Competition in ecological communities occurs when organisms use the same limited resources, reducing the amount available to one another. This interaction can affect survival, reproduction, population growth, and which species coexist within an ecosystem.
Plants compete for sunlight and nutrients, while animals may compete for food, territory, shelter, or mates. Studying these interactions is an important part of the science of biology because competition helps explain how organisms respond to resource shortages and how ecological communities develop.
Competition also influences the distribution of species, the size of their populations within ecosystems, and the traits that may become more common through natural selection.
What Is Ecological Competition?
Ecological competition is an interaction in which organisms negatively affect one another because they depend on a shared resource that is insufficient to meet everyone’s needs.
Competition differs from predation because neither competitor must consume the other. Instead, one organism’s use of a resource can reduce its availability to another organism.
Competition is one of several ecological interactions between species, alongside relationships such as predation, parasitism, and mutualism.
Its effects can also influence population dynamics in biological systems by changing birth rates, survival, and the number of individuals an environment can support.
Why Organisms Compete for Resources
Competition develops when organisms have overlapping resource requirements and the available supply becomes limiting. Resources may be abundant during one season but scarce during another.
Common resources involved in competition include:
- Food: Predators, herbivores, and other consumers may depend on overlapping food sources.
- Water: Plants and animals may compete when rainfall or freshwater supplies are limited.
- Sunlight: Taller plants can shade smaller plants growing nearby.
- Soil nutrients: Plant roots may absorb the same limited supplies of nitrogen, phosphorus, and other minerals.
- Space and shelter: Organisms may require particular nesting sites, territories, or attachment surfaces.
- Mates: Individuals of the same species may compete for reproductive opportunities.
Competition is not equally intense under all conditions. When resources are plentiful, organisms may experience little measurable competition even when their requirements overlap.
Two Main Types of Ecological Competition
Ecologists distinguish competition between members of the same species from competition between different species. Both can affect individual success and community structure.
Intraspecific Competition
Intraspecific competition occurs between individuals belonging to the same species. Because these organisms often have similar requirements, they may compete strongly when resources become scarce.
Examples include deer competing for winter forage, trees of the same species competing for light, and birds competing for nesting territories.
As a population becomes more crowded, competition may reduce average growth, reproduction, or survival. This is one mechanism through which population density can influence population growth.
Interspecific Competition
Interspecific competition occurs between individuals of different species that use overlapping resources.
For example, two bird species may feed on the same insects, or different grass species may absorb water and nutrients from the same soil.
The strength of competition depends on resource availability, how extensively the species’ needs overlap, and whether either species can use alternative resources.
Direct and Indirect Competition
Competition can also be classified according to how organisms interfere with one another’s access to resources.
Interference Competition
Interference competition occurs when one organism directly prevents another from accessing a resource. It may involve aggressive behavior, territorial defense, or other forms of obstruction.
For example, a territorial bird may chase other birds away from a feeding area. Some plants also release chemicals that inhibit neighboring plants, although demonstrating competition requires showing that the interaction affects resource access or performance.
Exploitative Competition
Exploitative competition occurs when organisms reduce a shared resource through consumption or uptake, without directly confronting one another.
Two plant species absorbing water from the same soil provide a common example. As one species takes up water, less may remain available for the other.
This form of competition can occur even when competing organisms never come into physical contact.
A Practical Example: Seedlings Competing for Sunlight
Imagine planting two identical trays of seedlings. One tray contains five seedlings spaced widely apart, while the other contains twenty seedlings growing in the same amount of soil.
If both trays receive similar light, water, and nutrients, the crowded seedlings may begin shading one another as their leaves expand. Their roots may also compete more strongly for water and minerals.
Under these conditions, some crowded seedlings may grow taller while developing less leaf area or lower total biomass than seedlings with more space. The exact outcome depends on the species and growing conditions.
This scenario illustrates intraspecific competition and shows why plant density can affect growth even when all individuals belong to the same species.
The Competitive Exclusion Principle
One of the most important ideas in the study of ecology is the competitive exclusion principle.
The principle predicts that two species competing for exactly the same limiting resources under stable environmental conditions cannot coexist indefinitely if one consistently has a competitive advantage.
Over time, the stronger competitor may exclude the other from that particular habitat. Competitive exclusion does not mean the weaker species must become extinct everywhere.
Real ecosystems are often more complicated than this simplified prediction. Environmental variation, predators, disturbances, and differences in resource use can allow competing species to coexist.
How Resource Partitioning Allows Species to Coexist
Resource partitioning occurs when species use different portions of a resource or obtain resources in different places or at different times.
Species occupying slightly different ecological niches within communities may experience less direct competition because their resource requirements overlap less extensively.
Examples of resource partitioning include:
- Bird species searching for insects at different heights within the same forest.
- Predators hunting at different times of day.
- Herbivores feeding on different plant species or plant parts.
- Plants developing roots at different soil depths.
These differences can reduce competition, although species may still compete for some shared resources.
How Competition Influences Natural Selection
Competition can influence natural selection when inherited traits affect an organism’s ability to obtain resources, survive, or reproduce.
For example, if birds with particular beak shapes obtain scarce seeds more efficiently, those individuals may have greater reproductive success under certain environmental conditions.
Across generations, inherited traits associated with greater reproductive success may become more common within a population. This is one way competition can contribute to evolutionary change.
Competition can also contribute to character displacement, in which competing species show greater differences in resource-related traits where they coexist than where they occur separately.
Competition Across Different Ecosystems
The resources involved in competition vary among habitats, producing different ecological outcomes.
- Forests: Trees compete for sunlight, soil moisture, and nutrients. Dense canopies can limit the growth of plants below.
- Grasslands: Grasses compete for water and nutrients, while grazing animals may depend on overlapping food supplies.
- Marine ecosystems: Sessile organisms may compete for attachment space, while plankton-feeding animals can use shared food sources.
- Deserts: Plants may compete for limited soil moisture, particularly after infrequent rainfall.
Competition can become stronger or weaker as seasons change, populations fluctuate, and environmental disturbances alter resource availability.
How Competition Shapes Community Diversity
Competition influences which species become abundant, where organisms live, and how resources are distributed among members of a community.
Strong competition can reduce local diversity when one species excludes another. In other circumstances, differences in resource use and environmental conditions allow multiple competitors to persist together.
Competition therefore does not automatically increase biodiversity or maintain ecological balance. Its effects depend on the species involved, resource limitations, environmental variation, and interactions with predators and other organisms.
Frequently Asked Questions
What is the difference between intraspecific and interspecific competition?
Intraspecific competition occurs between individuals of the same species, while interspecific competition occurs between different species. Both involve overlapping resource needs and can reduce growth, survival, or reproduction.
Can two competing species live in the same ecosystem?
Yes. Competing species can coexist when they use resources differently, experience changing environmental conditions, or are influenced by other ecological interactions that prevent one species from consistently excluding the other.
Does competition always involve fighting?
No. Many organisms compete indirectly by consuming or absorbing shared resources. Plants drawing water from the same soil can compete without physically confronting one another.
Competition in ecological communities affects how organisms obtain resources, how populations grow, and which species can coexist. Intraspecific and interspecific competition can lead to reduced growth, competitive exclusion, or differences in resource use that support coexistence. Understanding these outcomes helps explain the structure and changing diversity of natural ecosystems.
Sources: Begon, Townsend, and Harper, Ecology: From Individuals to Ecosystems; Ricklefs and Relyea, The Economy of Nature; OpenStax, Biology 2e.
